The present invention relates to a method of and an apparatus for aligning sheets advancing in an overlapping array in general, and more particularly to such a method and apparatus which are used for feeding properly aligned sheets of the above-mentioned array to a printing machine.
There are already known various arrangements for feeding sheet material to be printed to a printing machine, among them such which are operative for feeding individual sheets, previously arranged in a stack, to the printing machine. While it has been proposed to individually lift the respective sheets from the stack and individually convey them to the printing machine, experience has shown that the speed of operation of the printing machine can be dramatically increased when, rather than separately conveying the individual sheets, such sheets are advanced to the printing machine in the form of an array of such sheets in which each succeeding sheet partially overlaps the preceding sheet. When this approach is resorted to, however, it has been found that many problems arise, particularly those connected with proper engagement of the individual sheets by devices which align each individual sheet of the succession of sheets along its lateral edge. However, a proper alignment of each individual sheet both along its front edge and along its lateral edge is a necessary precondition for the proper performance of the printing function by the printing machine. Thus, the problems which have been encountered previously in connection with reliably aligning individual sheets along the lateral edges have so far hampered the full-scale introduction of this expedient in the printing industry.
In an attempt to eliminate these problems, it has already been proposed to utilize a suction rail or a special arrangement at the in-feed gripper of the printing machine which laterally or transversely displaces the individual sheet relative to the array either prior to or during its introduction into the printing machine. This transverse displacement of the individual sheet relative to the array of such sheets exposes the lateral edge of the individual sheet to be aligned for the alignment of the individual sheet along its exposed lateral edge.
However, experience has shown that even this attempted solution to the existing problem is disadvantageous in some respects, particularly in view of the fact that the lateral displacement and alignment must be finished by the time when the immediately succeeding sheet, and actually the front edge thereof, reaches the operating range of the pulling device which displaces the individual sheets in the lateral direction. As a result of this, only so much time is available for the alignment of the individual sheet by the front stops and by the lateral guides as there is between the arrival of the individual sheets to be aligned to the front stops and the arrival of the next-succeeding sheet of the array into the operating range of the lateral displacing device. Such a time period is extremely short, particularly at high feeding speeds, so that a reliable alignment of the individual sheets cannot be always accomplished, especially for some sorts of paper sheets or the like.